Antenna with side lobe absorber mounted adjacent thereto



y 1965 R. G. LEFFELMAN ETAL 3,

ANTENNA WITH SIDE LOBE ABSORBER MOUNTED ADJACENT THERETO Filed Dec. 14, 1959 FIG.2.

INVENTORS RICHARD G. LEFF'ELMAN JOHN TORRISI PRIOR ART FIG. 3.

3,196,442 ANTENNA WITH SllDE [ASHE ABSQRBER MQUNTED ADJACENT THERETQ Richard G. Lefielman, Overland, and John Torrisi, Florissant, Mo, assignors to McDonnell Aircraft Corporation, St. Louis, Man, a corporation of Maryland Filed Dec. 14, 1959, Ser. No. 859,255 4 Claims. (Cl. 343765) The present invention relates generally to devices for shaping the radiation pattern of an antenna and more particularly to radar antenna radiation pattern shaping means.

Various means have been devised in the past to eliminate or at least to reduce antenna radiation in a particular direction. The known means have included radiation absorbing devices which are mounted adjacent to the antenna to absorb radiation in a chosen direction. In the known constructions the radiation absorbing material has been fixedly attached to an immovable object adjacent to the antenna such as a radome but has not been satisfactory for use with moving antenna structures such as the antenna structures employed with radar gear because movement of the antenna changes the relative position of the absorbing material with respect to the antenna and thereby changes the direction of maximum radiation absorption relative to the radiation pattern. If the antenna is moved through a large enough angle the main antenna beam may actually be directed at the absorbing material and be undesirably attenuated While the (undesirable) side radiation which it is intended to absorb is unaffected. Another major problem of known radiation absorbing elements has been their inability to withstand intense radiation resulting in a functional breakdown when located in a strong electromagnetic field. These and other disadvantages of the known devices are overcome by the present devices which comprises an antenna structure having a feed element or radiator, a refiector located adjacent to said radiator, and means mounted on the antenna structure for attenuating radiation therefrom in a chosen direction relative to the antenna.

It is a principal object of the present invention therefore to provide means for reducing antenna radiation in a chosen direction, such as the extreme side lobes of the radiation pattern so that the effective radiation is concentrated as desired.

Another object is to uniformly reduce antenna radiation in a chosen direction relative to the antenna regardless of the direction in which the antenna is facing.

Another object is to maintain a constant relationship between a main antenna radiation beam and a secondary or side lobe.

Another object is to provide an inexpensive, lightweight and compact device capable of absorbing antenna radiation, which device is capable of withstanding large amounts of radiation without breaking down and which can be installed on new or existing antenna structures with minimum effort.

Another object is to provide a radiation absorbing device for antennae which reduces radiation therefrom in a chosen direction without reducing, distorting or bending the radiation in other directions.

Another object is to provide a radiation absorbing device which can be mounted on antenna structures and moved therewith.

Still another object is to increase the accuracy of radar antennae and the like by maintaining a constant fixed relationship between the magnitude of the principal radiation lobe and the side lobes.

These and other objects and advantages of the present invention will become apparent after considering the United States Patent "ice following detailed specification in conjunction with the accompanying drawing.

In the drawings:

FIG. 1 is a perspective view of an antenna structure having pattern shaping means constructed according to the preferred teachings of this invention;

FIG. 2 is a cross-sectional elevational view on an enlarged scale taken along the center line of the antenna of FIG. 1 and showing in phantom outline a typical attenuated radiation pattern therefor; and

FIG. 3 is a cross-sectional view of an antenna mounted in a radome for movement about different axes and relative to a radiation absorbing means which is illustrative of prior art examples.

Referring to the drawing more particularly by reference numbers, the number It) refers to an antenna structure constructed according to a preferred teaching of the present invention. The antenna 10 has a parabolic reflector or shield 12, a radiator 14 mounted centrally adjacent the concave side of the reflector 12, and suitable leads 16 connecting the radiator 14 to a radar set 18 or similar radiation transmitting or receiving device. The reflector 12 is movably carried on a universally pivoting device A.

When energy is fed to the radiator 14 from the radar set 18 it is radiated therefrom in a pattern, as in FIG. 2, the characteristics of which depend upon the characteristics of the antenna components including the radiator and the reflector, the characteristics of the signal itself, and also the characteristics of the surrounding structures and their location relative to the antenna. For example, the forward lobe 20 emitted by the radiator 14 has a plurality of secondary or side lobes which are conveniently shown as envelope 22 and 26 in which the secondary and all side beams are lumped together. In the more generalized situation there will be a number of such side lobes, but for the sake 'of this invention these side lobes are dealt with as one general side lobe, which forms the envelope of all side lobes.

In order to reduce the magnitude of the side lobe in a given direction or portion of the annular envelope, depicted in FIG. 2, a stick 24 of radiation absorbing material is mounted adjacent to the radiator 14 as shown in FIGS. 1 and 2. Any suitable radiation absorbing material can be used for this purpose provided it can withstand the intensity of the radiation without breaking down. I-ieretofore, the absorber material 28 of FIG. 3 has consisted of a section of foam rubber with graphite therein and having a ply of aluminum foil on one surface. This is generally referred to as harp material and may have other material components. A greatly more effective material which has been found particularly desirable because of its electromagnetic absorption characteristics is ferrite. Ferrite is an amorphous substance, usually although not necessarily, an iron compound having magnetic characteristics. When a rod of ferrite is mounted as described, the radiation in the unwanted direction is absorbed and the radiation pattern from the antenna is modified from a pattern having approximately equal side lobe radiation in a band about the radiator 14 to that shown in FIG. 2. In the example of FIG. 2, the radiator 14 has the generally cylindrical shape illustrated with the outer free end defining an imaginary plane just within the rim opening of the reflector, and the stick has its free end at or to the rear of the imaginary plane for the radiator. The pattern of radiation when the stick 24 is used consists of the main beam 20 which is substantially unchanged, and a side lobe 22 which is modified from a uniform annular band by having its intensity greatly reduced in the direction of the stick 24, as at 26. The shape of the side lobe 22 is unchanged in the upper and sidewise directions but is substantially attenuated in the down ward direction. In actual tests it has been found that the reduction in radiation in the downward direction 'is very substantial using a ferrite stick, and that the effect on'the main beam 20 even" when theradiatio n is relatively high is negligible. g

A particular application where the present device has been found useful is in connection with the radar systems employed on' airplanes. 'In such devices not only is it important to properly modify the radiation pattern in given directions but it is also important that the size and 7 'radar set, the radar antenna main beam is directed to a remote target; The main beam thus directed, is reflected from the target and picked upby the same antenna. At

tbe same time energy is also being radiated in the form of" side lobes, and part of the side lobe energy is radiated downwardly and reflected from the earths surface. "The "reflected energy received from the target area, and from the earths surface as a resultof the side lobe energy, is

a line which is referred to as the altitude line and which "extends across the 'faceof the scope (as if it were a target at a range corresponding to the aircraft altitude, and as -a target or bright spot.. As the aircraft closes on the) ference in intensity'will insure. that the radar remains locked on the targetas the target passes through the alti' tude line. In this regard it should be noted that the powerdue to noise"and'earthreflections is due in large part -to the strength of the side lobe reflections; If the reflected energy due to the side lobes is large, the power which contributes to the altitude line will be large and it will'be more difficult for the tracking radar to distinguish between the altitude line and target and lock on. may be lostas the target passes through the altitude line.

When thiscondition exists it becomes diificult if not i inpossible to successfully track. f c

As previously noted, toinsure satisfactory tracking, the

-power received'from the target signal must be appreciably greater than the power received from the earths signal. The ratio of the power received from the earth and the power received from the target depends on the reflecting areas of the reflecting bodies and the gain'of the antenna when power is received froma particular direction. The ratio of the observed area of the earth to the observed target area is a factor which cannot be controlled. There, fore, in order to have an appreciably stronger'signal from the target than from the earth, some means must be de vised to reduce theaniount ofenergy'radiated in and received from the side lobes without reducing the energy directed by the main beam at .the target.

By using a ferrite rod24 mounted adjacent to'the lower side of the radiator (FIG. 2) the desirable reduction in downward radiation is etfected. The same effect is obtained with'the harp material 28 in FIG. 3, howevenin" the past, attempts to solve this problem have been unsuccessful because they have not made provisions taking into account the movement of the antennaas it follows the .target. .tion absorbing material'28 is mounted in a fixedlposition relative to the antenna 10a mounted on the universally pivoting device A within the radome 30 of an airplane. The antennalfla works satisfactorily during level flight sassy ass For example, as illustrated in FIG. 3, the radiabut not otherwise because during certain flight attitudes the antenna ltla will actually be looking through the absorbing material 28 or else looking away from the absorbing material, as indicated in the broken line positions.

The present invention overcomes these disadvantages by mounting the ferrite rod 24, or the absorbing material 23 adjacent to. the radiator itself on the antenna so that r it moves with the antenna and not relative thereto. Ob-

viously, therefore, regardless of the attitude of the airplane 1 or the direction in which the antenna is facing, the antenna never faces into or away from the absorbing material. The purpose of applicants device then is to absorb or redirect the side lobe power that otherwise wouldbe radiated toward the earth or some other body so that the reflected [energy received by the main lobe will always be substantially greater than the energy 'received from the side lobes. In a particular test it was discovered that a ferrite rod 7 inches long and .250 inch in diameter reduced side lobe radiation by at least 4 decibels in the downward direction, 'and at the same time produced no significant attenuation, distortion, or bending in the azimuth plane of the main :beam. This indicates that the ferrite rod produced no deterioration of themain or target beam but effect substantial reduction in the downward side lobe radiation.

Temperature test also indicated that the ferrite rod would not break down even' under extremeoperation conditior'is. Tests have also been conducted satisfactorily over land and over water; V V While'the present invention has been described in connection with a specific applicationand in a specific antenna construction, it is obvious that it can als'o'be used with many other antenna structures and for many other appli- 1 cations without departing from the spirit and scope of the invention. Furthermorejmany changes, alterations and modifications of the'pres'ent device as well as other uses and applications will ,be' apparent to thoseskilled in the art; All such changes, alterations, modifications and uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the init) vention which is limited only by the claims which follow.

What is claimed is: j a V 1. In an antenna having a reflector shield and a radiator mounted forwardly thereof in the direction of principal radiation'and capable of radiating a forward radiation lobe and a side lobe, the'improveme'nt comprising a member constructed substantially entirely of ferritematerial 'mourited laterally adjacent to the radiator on one side thereof and spaced forwardly of said shield'in position to absorb jside lobe radiations from the radiator substantially onlyin the direction of said ferrite member.

2. Means for'clianging the radiation pattern of an antenna comprising an antenna having a parabolic reflector ishield, a radiator mounted adjacent to said shield in the direction of principal radiation therefrom, means conjnected to said radiator for energizing said radiator to generate a signal therefrom'having a forward radiation lobe -and'at least one sidelobe, and a radiationshaping member mounted on said antennaspaced forwardly of said shield and adjacent to the side of said radiator in the path of said radiation side lobe, said member being constructed of a ferrite substance capable of reducingby absorption the. side lobe radiation in the direction thereof.

3. In a directional antenna having a reflective shield and a radiator member mounted thereon adjacent to and s5forwardly of said shield, said radiator member being capable of radiating a main forward energy beam and side dobeenergy in a direction angularly'related to the forward -beam, the improvementcomprising a radiation absorbing element mounted on the antenna adjacent to a side of said -radiator and spaced forwardly of the, shield in the path of the side lobe energy,"said element being constructed 7 substantially entirely of an amorphous substance having magnetic characteristics capable of absorbing side lobe radiations from the radiator substantially only in the direction thereof, and means for angularly moving said antenna without changing the relative positions of said shield, said radiator and said radiation absorbing element.

4. In a target seeking radar antenna, means for attenuating disturbing radiations and reflections comprising a movable reflector which projects radiations and receives reflections from frontal areas, a radiator-receiver element mounted on and positioned ahead of said reflector in position to project and receive frontal radiations and reflections including a principal forward energy beam and side lobe radiations and reflections, and absorber means positioned ahead of the reflector in the path of said side lobe radiations and reflections, said absorbing means including an element constructed substantially entirely of ferromagnetic material capable of absorbing side lobe radiav tions and reflections in the direction thereof without producing any substantial effect on the radiations and reflections of the principal forward beam.

References Cited by the Examiner UNITED STATES PATENTS 2,597,313 5/52 Gardner 343-840 2,750,497 6/56 Stott 343-788 2,808,586 10/57 Chrlich et a1 343-838 2,923,934 2/60 Halpern 343-l8 2,926,349 2/60 Jensen 343-840 2,982,962 5/61 Kendall et al 343-840 2,998,602 8/61 Cacheris 343-782 X HERMAN KARL SAALBACH, Primary Examiner.

GEORGE N. WESTBY, ELI LIEBERMAN, Examiners. 

1. IN AN ANTENNA HAVING A REFLECTOR SHIELD AND A RADIATOR MOUNTED FORWARDLY THEREOF IN THE DIRECTION OF PRINCIPAL RADIATION AND CAPABLE OF RADIATING A FORWARD RADIATION LOBE AND A SIDE LOBE, THE IMPROVEMENT COMPRISING A MEMBER CONSTRUCTED SUBSTANTIALLY ENTIRELY OF FERRITE MATERIAL MOUNTED LATERALLY ADJACENT TO THE RADIATOR ON ONE SIDE THEREOF AND SPACED FORWARDLY OF SAID SHIELD IN POSITION TO ABSORB SIDE LOTE RADIATIONS FROM THE RADIATOR SUBSTANTIALLY ONLY IN THE DIRECTION OF SAID FERRITE MEMBER. 